Due to the tremendous success of the Internet, it has become a challenging objective and task to make use of the Internet Protocol (IP) over all kinds of communication links, including wireless links. One particular challenge is using Internet Protocol for wireless communications. Wireless communication is typically performed by employing devices which are variously known as mobile stations, mobile telephones, and wireless mobile telephones (which are herein referred to by the more generic term wireless terminals), and which typically communicate using a relatively narrow bandwidth. On the other hand, Internet Protocol is somewhat bandwidth inefficient because a typical information transmission may have a significant portion of the transmitted information related to administrative overhead rather than substantive data.
Internet Protocol enables transfers of information in packets from one point (or electronic device) to another point (or electronic device) through a web of networks without using a predetermined or dedicated path. Internet Protocol packets include a header containing administrative information such as routing information (e.g., destination address), and a payload containing the substantive information (e.g., digital data from an application software or digital audio data from a conversation). When ordinary audio data, such as speech or voice information, is transported by the ordinary protocols such as Internet Protocol (IP), user datagram protocol (UDP), and realtime transport protocol (RTP), the header may represent about 70% of the packet. Having such a large portion of the packet taken up by header information will result in a very inefficient usage of the link. In this example, only approximately 30% of the used bandwidth is allocated to substantive information. Therefore, it is a particularly difficult task with many wireless links that have narrow bandwidth, e.g., wireless terminals with cellular links, to achieve efficient transmission because that large headers are employed by Internet Protocol.
Transporting audio data using Internet Protocol is particularly challenging because audio information, e.g., speech or voice information, compared to many other types of data, is particularly sensitive to delays. Accordingly, the quality of words being transmitted using Internet Protocol (i.e., voice over IP (VoIP)) may be reduced by the amount of space in a packet taken up by the header since the header reduces the amount of audio information per packet. One way developed to address the large header sizes employed in Internet Protocol is known as header compression.
Header compression (HC) minimizes the necessary bandwidth for information carried in headers on a per-hop basis (i.e., from one device to another in a chain of devices making up a network) over point-to-point links. The header compression techniques for reducing the header size, in general, have a long history of use within the Internet community. Several commonly used header compression protocols include Internet Engineering Task Force (IETF) request for comment (RFC) 1144 (Van Jacobson (VJ)), RFC 2507 (Internet Protocol Header Compression (IPHC)) and RFC 2508 (compressing realtime transport protocol (CRTP)). Header compression takes advantage of the fact that some fields in the headers are not changing from packet to packet within a particular flow of transmitted information, or change with small and/or predictable values. Header compression schemes make use of these characteristics and send static information only initially, while changing fields are sent with their absolute values or as differences from packet to packet. Completely random information has to be sent without any compression at all. One can usually consider a header compression scheme as some type of a state machine and the challenging task is to keep the compressor and decompressor states, called contexts, consistent with each other, while keeping the header overhead as low as possible.
Header compression is thus an important component to make Voice over IP (VoIP) over Wireless (VoIPoW) an economically feasible alternative to circuit switched voice. Header compression solutions for this purpose have been developed by the Robust Header Compression (ROHC) Working Group of the IETF. Given today's communication and electronics technology, using VoIPoW without header compression is unlikely to provide a reasonable audio quality for most VoIP uses. However, header compression processing takes processing time and power which is usually a concern in wireless terminals, for example mobile telephones, that are often small hand-held devices that operate on battery power. Therefore, although header compression is likely to be used for VoIP communications for the foreseeable future it does have some drawbacks for VoIPoW.
In existing second generation wireless radio telephony, most of the wireless network's equipment, particularly the wireless terminals and base stations, do not have VoIP capability. Rather, they operate using other protocols prevalent with present generation wireless telephones, for example, Global System for Mobile communications (GSM), Personal Digital Communications (PDC), Code Division Multiple Access (CDMA), or Wideband CDMA (WCDMA). However, due to the popularity of Internet Protocol in fixed networks, it is desirable to implement VoIP in wireless networks. The focus for the future is to implement end-to-end VoIP communication as part of an all Internet Protocol network. However, to implement VoIP will require changes and migration to both network hardware and software in such areas as base stations, and changes to wireless terminals to support VoIP. These changes will be difficult to coordinate, time consuming, and expensive. Therefore, a more timely, cost effective, and easy to implement interim standard to make interim implementations interoperable with other implementations at least in part compliant to existing Internet Protocol standards would be useful.